Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and a rotor. The rotor is coupled to the nacelle and includes a rotatable hub having one or more rotor blades. The rotor blades are connected to the hub by a blade root. The rotor blades capture kinetic energy from wind using known airfoil principles and convert the kinetic energy into mechanical energy through rotational energy to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
The particular size of the rotor blades is a significant factor contributing to the overall capacity of the wind turbine. Specifically, increases in the length or span of a rotor blade may generally lead to an overall increase in the energy production of a wind turbine. Accordingly, efforts to increase the size of rotor blades aid in the continuing growth of wind turbine technology and the adoption of wind energy as an alternative and commercially competitive energy source. Such increases in rotor blade size, however, may impose increased loads on various wind turbine components. For example, larger rotor blades may experience increased stresses at the connection between the blade root and the hub, leading to challenging design constraints, both characterized by extreme events and fatigue life requirements.
Various proposals have been made for improving the blade root. For example, U.S. Pat. Pub. No. 2013/0111752 describes a problem with conventional wind turbine blades made of fiber reinforced plastic wherein the prevailing reinforcement fibers run in a longitudinal direction of the rotor blade, resulting in an “ovalization” of the root (also called root ring) once the blade is removed from the molding tool and is subjected to gravity forces without support at the root end. The '752 Publication proposes to assemble supporting rods with an interface section to a hub interface of the wind turbine in an essentially circular shape such that there are gaps between the supporting rods. Fibers are arranged in the gaps. A first molding tool is placed along an outer surface of the circular shape and a second molding tool along an inner surface of the circular shape. A resin is then injected between the supporting rods, which function as a support for the fibers and a structure for the interface section to the hub.
U.S. Pat. Pub. No. 2014/0119926 describes a cylindrical blade root section defined by an inner circumferential component and an outer circumferential component separated by a radial gap. A ring insert is disposed in the radial gap and is bonded to the inner and outer circumferential components. The ring insert has an inner circumferential surface and an outer circumferential surface, wherein at least one of these surfaces has a span-wise or circumferentially varying cross-sectional profile that increases the bonding surface area as compared to a constant or non-varying cross-sectional profile.
U.S. Pat. Pub. No. 2012/0315145 describes a blade root with cylinder-segment connecting members having stud-like connecting elements inserted in between respective fibers and/or the casting material of the rotor blade while moulding. The connecting members act as connecting means for aligning the connecting elements in a given geometrical relationship. Generally, the respective connecting members provide the blade root with additional mechanical stability, i.e. in particular stiffness so that ovalisation or deformation effects are avoided or at least reduced. The respective connecting members are adapted to transfer and/or distribute respective externally applied loads into the blade root structure of the rotor blade. The connecting elements may be integrally built with a respective connecting member or mounted to a respective connecting member.
A Publication from Sandia National Labs (SAND2003-0719 Unlimited Release Printed May 2003; http://windpower.sandia.gov/other/030719.pdf) includes a description of a method for installing root studs in a blade by directly embedding the studs within the laminate material to reduce the number of manufacturing process steps and tooling requirements. Dry fabric is rolled around each stud and folded material is placed between the studs prior to resin infusion.
U.S. Pat. No. 8,172,538 describes a method of manufacturing a wind turbine blade shell member having an incorporated fastening member near the root. The fastening member is positioned in a mold with pre-fabricated sticks surrounding a substantial longitudinal part of the fastening member prior to resin infusion. A guiding means is provided for aligning the fastening member relative to a further fastening member and/or relative to the mold during molding.
Thus, the industry is continuously seeking to provide a method for manufacturing wind turbine blade root sections with uniform structural properties in a cost-effective and timely fashion as compared to existing techniques.